Estimation of greenhouse gas generation in wastewater treatment plants – Model development and application M. Bani Shahabadi, L. Yerushalmi, F. Haghighat * Department of Building Civil and Environmental Engineering, Concordia University, 1455 de Maissonnuve Blvd. West Montreal, QC, Canada H3G 1M8 article info Article history: Received 10 June 2009 Received in revised form 17 December 2009 Accepted 18 December 2009 Keywords: Greenhouse gas Wastewater treatment Food processing On-site and off-site emissions Hybrid process abstract A comprehensive mathematical model has been developed to estimate greenhouse gas (GHG) emissions by wastewater treatment plants (WWTP) resulting from on-site and off-site activities. The contribution of individual processes to the production of GHGs in a typical hybrid treatment system for food processing wastewaters has been determined. The results show that the recovery of biogas and its reuse as fuel have a remarkable impact on GHG emissions and reduce the overall emissions by 1023 kg CO 2 ed 1 from a total of 7640 kg CO 2 ed 1 when treating a wastewater at 2000 kg BOD d 1 . Furthermore, the recovery of biogas and its combustion may be used to cover the entire energy needs of the treatment plant for aer- ation, heating and electricity generation while creating emissions credit equal to 34 kg CO 2 ed 1 . The off- site GHG emissions resulting from the manufacturing of material for on-site usage were identified as the major source of GHG generation in hybrid treatment systems. These emissions account for the generation of 4138 kg CO 2 ed 1 , or 62% of the overall GHG emissions when biogas recovery is carried out. The inclu- sion of GHG emissions from nutrient removal as well as off-site processes in the overall GHG emissions of WWTPs increased the accuracy and completeness of this estimation, lending support to the novelty of the present study. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction During the last two centuries, human activities such as the pro- duction and consumption of fossil fuels, as well as agricultural and industrial activities have caused an increase in the atmospheric concentration of harmful greenhouse gases (GHG), particularly CO 2 , CH 4 , and N 2 O(El-Fadel and Massoud, 2001). The increase in atmospheric GHG concentration has led to climate change and glo- bal warming effect, motivating international efforts such as the Kyoto Protocol to reduce GHG emissions. The contribution of a greenhouse gas to global warming is commonly expressed by its global warming potential (GWP) which enables the comparison of global warming impact of the gas and that of a reference gas, typically carbon dioxide. On a 100-year basis, the GWP of carbon dioxide, methane and nitrous oxide are 1, 23 and 296, respectively (IPCC, 2001). Wastewater treatment plants (WWTPs) are recog- nized as one of the larger minor sources of GHG emissions (Sahely et al., 2006). WWTPs produce CO 2 , CH 4 , and N 2 O during the treatment processes and CO 2 from the energy demand of the plant. Off-site GHG emissions result from several activities that in- clude the production of electricity as well as the production and transportation of fuels and materials for on-site consumption. These emissions are traditionally assigned to the energy sector in- stead of the end-user which is the WWTP. Besides, under the IPCC protocols, CO 2 production from biogenic sources should not be considered as GHG emissions. This assumption is true if biomass or renewable fuels are produced without fossil fuel input, which is rarely met in reality. These assumptions, along with the use of the IPCC emission factors, result in under-estimation of GHG emis- sions from WWTPs. In addition, most attempts to estimate GHG emissions by WWTPs have focused on the contribution of carbona- ceous biochemical oxygen demand (CBOD) to GHG emissions with- out considering the impact of nutrient removal (El-Fadel and Massoud, 2001; Keller and Hartley, 2003; Cakir and Stenstrom, 2005; Monteith et al., 2005; Sahely et al., 2006; Wei et al., 2008). The implementation of abatement strategies to reduce GHG emissions by WWTPs will further enhance their role in the protec- tion of our environment while preventing the imposition of carbon taxes on these industrial facilities. In order to achieve this objec- tive, a comprehensive mathematical model has been developed to provide a thorough estimation of GHG emissions resulting from on-site and off-site activities of WWTPs. The inclusion of off-site GHG emissions along with the impact of nutrient removal activi- ties provides a more accurate assessment of these harmful emis- sions. The application of the developed model was demonstrated in the estimation of GHG emissions from a hybrid treatment 0045-6535/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2009.12.044 * Corresponding author. Tel.: +1 514 848 2424x3192; fax: +1 514 848 7965. E-mail addresses: m_banish@encs.concordia.ca (M. Bani Shahabadi), laleh@encs. concordia.ca (L. Yerushalmi), haghi@bcee.concordia.ca (F. Haghighat). Chemosphere 78 (2010) 1085–1092 Contents lists available at ScienceDirect Chemosphere journal homepage: www.elsevier.com/locate/chemosphere